FIG. 1 illustrates a typical coil spring 10 which is widely used as a chassis spring in independent suspension systems of vehicles, and as may be seen, the coil spring 10 has a maximum diameter d at the medial portion of the barrel and converges axially from the medial portion to the ends thereof so as to have a generally barrel-shaped configuration. In such a barrel-shaped coil spring, the wound portion of the coil wire which converges toward each end to form a frusto conical configuration is called a pig tail. Various methods have been proposed heretofore to manufacture barrel-shaped coil springs having respective pig tails at the opposite ends. However, all of them disadvantageously require increased forming steps or fails to wind the coil spring with desired pitches, which can result in variations in the products. Specifically, it is difficult to shape the whole coil spring in but a single step while using a mandrel, because the opposite ends of the barrel-shaped coil spring are formed so as to have a frusto conical configuration. Another known method is to manufacture barrel-shaped coil springs without using a mandrel, wherein the coil wire is held by a pair of opposed supporting rolls and associated pressing rolls, and the position of the supporting rolls is variably controlled relative to the pressing rolls. In such a method, however, it will be noted that desired pitches can not be obtained, which disadvantageously tends to cause variations in the products.
Thus, the prior art method has been unsuccessful in forming a barrel-shaped coil spring in but a single step, and employed two steps. Here, an intermediate coil spring with a pig tail at one end is first manufactured by known means including a mandrel, as shown in FIG. 2, and thereafter, in a second step, the intermediate coil spring is formed to provide a second pig tail at its other end. FIG. 3 illustrates such a method of forming a pig tail at the other end of the intermediate coil spring, and as may be seen, the initial position X for forming a pig tail on the coil spring 10 is held by a first clamping means 12, and the free end E of the coil spring 10 is clamped by a second clamping means 14, which is rotated toward the coil center C along a locus depicted by the arrow. In this prior art method, however, the number of turns of the pig tail to be formed has been limited to one, namely, a maximum of 360 degrees in terms of rotational angle. Thus the prior art has failed to meet the specific production requirements for various pitches and spring constant of barrel-shaped coil springs. Further, since pig tails have been formed in a free condition without using a jig such as a mandrel, the finish accuracy of products has been inconsistent.